In one embodiment, a compound semiconductor vertical FET device includes a first trench formed in a body of semiconductor material, and a second trench formed within the first trench to define a channel region. Samples of the signals on the sub-bands are stored at intervals, the energy levels of the sub-bands are determined on the basis of the stored samples, a power threshold value is determined, and the energy levels of the sub-bands are compared with said power threshold value. It is noted that insulation of a gate insulating film of the first semiconductor layer transistor is almost equal with that of a gate insulating film of the second semiconductor layer transistor and the gate insulating film of the second semiconductor layer transistor is formed by means of radical oxidation or radical nitridation. An indicator may also be provided to further indicate to a user the expected media size, with media sensors being present to further avoid entry of incorrect media.